Photonic crystal formed by laser machining and having holes with sub-wavelength pitch

ABSTRACT

A method of laser machining using an ultra-fast pulse laser is presented. According to the present invention a plurality of holes with a pitch less than the wavelength of the laser are drilled into a material sample. Reliable and reproducible hole drilling is accomplished through an exemplary drilling sequence which applies a number of pulses at a first pulse energy to the surface spaced to avoid laser hardening of the surface for adjacent holes of a first set of holes. Next, the number of pulses is increased or the energy of the laser beam is increased to drill holes that are interstitial to the first set of holes. The exemplary laser machining process may used to produce both one-dimensional and two-dimensional photonic crystals, among other applications.

BACKGROUND OF THE INVENTION

[0001] Many emerging material processing applications in thesemiconductor and communications fields require sub-micron processingcapability. A number of competing technologies exist that either have,or will soon have, this capability, such as; electron beam etching,plasma etching, x-ray lithography, and machining with ultrafast pulselasers (laser machining). Of these technologies, only laser machiningprovides the advantages of operation in a standard atmosphere and insitu monitoring.

[0002] An important feature of ultrafast pulse lasers is theircapability to ablate surface regions smaller than their minimum,diffraction limited, spot size. This capability is created by thebrevity of the pulse, which allows for essentially no spreading of heatduring the pulse, and the Gaussian spatial beam profile. By carefullycontrolling the energy of a pulse, it is possible to raise the intensityof only a small region in the center of the beam above the ablationthreshold for the material being machined. Because of the lack of heatconduction in the pulse duration, only the small region is ablated. Inthis way, holes may even be laser machined with diameters less than thewavelength of the laser, for example holes having a diameter ofapproximately 500 nm may be drilled using a 775 nm femtosecond pulselaser. Geometrically speaking, it is possible to space these holes asclose as 500 nm. When the holes are drilled one by one from one end tothe other with the same laser, however, the previous method of lasermachining a series of holes, the hole center-to-center spacing (pitch)cannot approach this limit.

[0003] The following example illustrates this problem. Assume that thefirst hole is drilled with certain laser intensity and a certain numberof laser pulses. The laser intensity is chosen so that laser-inducedablation occurs only in the central portion of beam spot formed on thesurface, where the breakdown threshold is reached. This ablation leadsto hole drilling. Even though the surrounding area that is irradiateddoes not reach ablation threshold, however, it may undergo materialproperty changes that increase the ablation threshold for subsequentlaser irradiation. This phenomenon of laser irradiation-induced materialhardening, laser hardening hereinafter, means that using the same laserintensity and number of pulses on the hardened area, a new hole may notbe drilled in the laser hardened region. Therefore the hole-drillingreliability and reproducibility suffers. This issue is of particularimportance in a device, such as a photonic crystal, in which a largenumber of substantially identical holes placed with a precise sub-micronpitch are desired.

SUMMARY OF THE INVENTION

[0004] A solution to this problem is an exemplary laser machiningprocess of the present invention, which allows closer placement of theholes to reach sub-wavelength center-to-center hole spacing (pitch).

[0005] The first step of this exemplary process is to separate the holepositions on the surface of the material sample into two groups selectedso that no two members of either group have a pitch less than the laserbeam spot size. Next the pulse energy of the laser beam is set to apredetermined level, selected to drill holes of the desired diameter inthe surface. Then the sample is positioned so as to focus the laser beamon the surface at a hole position in the first group and a number oflaser pulses are applied to ablate the surface, thereby forming a holein the surface. The process is repeated for every hole position of thefirst group.

[0006] At this point the pulse energy of the laser beam is set to asecond predetermined level, selected to drill holes of the desireddiameter in the surface once it has been laser hardened. Then the sampleis positioned so as to focus the laser beam on the surface at a holeposition in the second group and a number of pulses of the laser beamare applied to ablate the surface, thereby forming a hole in thesurface. Alternatively, the pulse energy of the beam may be maintainedat the same level and a greater number of pulses applied to the laserhardened surface. The process is repeated for every hole position of thesecond group.

[0007] Alternatively, the laser beam may be moved rather than thesample.

[0008] Another aspect of the present invention is an exemplary photoniccrystal comprising a plurality of holes formed in a material sample bythe method described above.

BRIEF DESCRIPTION OF DRAWINGS

[0009]FIG. 1A is a top view drawing illustrating widely spaced holes andan associated laser hardened region on a section of laser machinedsample.

[0010]FIG. 1B is a top view drawing illustrating closely spaced holesand an associated laser hardened region on a section of laser machinedsample.

[0011]FIG. 2 is a block diagram illustrating an exemplary lasermachining apparatus of the present invention.

[0012]FIG. 3 is a top view drawing illustrating an exemplary photoniccrystal formed by an exemplary laser machining method of the presentinvention.

[0013]FIG. 4 is a top view drawing illustrating a laser machined groovecut in a section of material using an exemplary laser machining methodof the present invention.

[0014]FIG. 5 is a flowchart diagram showing an exemplary laser machiningmethod of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] This invention describes a method used in laser materialprocessing applications for drilling small holes, having substantiallyuniform size and shape which holes have a diameter that is less than thewavelength of the laser beam. Currently, many laser material processingapplications such as the machining of photonic band gap crystals usingfemtosecond laser pulses require sub-micron processing capability. FIG.1A shows a row of widely separated holes 102, which have been lasermachined into a material sample 100. These holes may be formed to have adiameter, d, less than the wavelength of the ultra-fast laser used todrill the holes. For example, if the laser used is a 775 nm femtosecondlaser, the holes in FIG. 1 may have a 500 nm diameter with a pitch, p,of 2 μm. Geometrically speaking, it is possible to space these holes asclose as 500 nm. However, when the holes are laser machined, the laserhardened regions 104 are also formed along the surface of the samplewith a diameter equal to the beam width, w. In the example, w is 1.5 μm.Because the material properties in the laser hardened regions may besignificantly different from those in the unhardened portions of thesurface, a hole drilled at least partially within the laser hardenedregions of the surface may have very different characteristics than onedrilled in an unhardened portion.

[0016]FIG. 1B illustrates an exemplary material sample in which lasermachined holes are closely spaced such that, if the holes are drilledone after another down the line, then each hole, after the first, haspart of its area, near intersections 106, formed in a laser hardenedregion of the surface. Using the same exemplary laser-beam parameters asabove with reference to FIG. 1A, the pitch, p, in FIG. 1B is 750 nm.

[0017] Holes formed in this manner may be incomplete or deformed, whichis undesirable, particularly in applications, such as photonic crystals,where precise tolerances are desired. Because the minimum diameter ofthe laser hardened region is determined by the diffraction limited beamspot size of the laser, laser machining substantially identical holeswith a sub-wavelength pitch using a sequential drilling pattern may notbe possible in many materials.

[0018]FIG. 2 illustrates an exemplary embodiment of a laser machiningapparatus used in the present invention. The ultra-short pulse laser204, for example a 775 nm femtosecond laser, generates a laser beam 200,which desirably oscillates on a TEM_(0,0) mode. The laser beam may bepropagated between the components of the laser machining apparatusthrough the intervening air or along an optical fiber (not shown). Afterleaving the laser, the beam passes through a variable intensityattenuator and shutter assembly 206, which contains a shutter (notshown) and a variable intensity attenuator (not shown). The shuttercontrols the number of pulses and the variable intensity attenuatorcontrols the pulse energy of the beam. A half-wave plate and crossedpolarizers may form an exemplary variable intensity attenuator. Withthis combination, it is possible to select a desired laser intensity andnumber of pulses to be applied. An exemplary embodiment of the inventionuses an ultra-short pulse laser having a wavelength of 775 nm. With thevariable intensity attenuator, the pulse energy of the laser may be setbetween 1 nanojoule and 1 microjoule.

[0019] The laser beam is then focused onto the surface of the sample 100to be machined. An output coupler 202, such as a long-working-distance,high numerical aperture microscope objective, may be employed to focusthe beam 200 onto the workpiece 100. The sample 100 may be attached to aprecise XYZ-translation-stage 210 with nanometer resolution. By movingthe XYZ stage 210 relative to the optical coupler 202, one may preciselyfocus the laser beam to any spot on the sample, as illustrated by thefocused laser beam 208 in FIG. 2.

[0020] An exemplary method of the present invention, charted in FIG. 5,is a procedure for drilling holes with sub-wavelength separationsemploying a laser machining apparatus of the sort illustrated in FIG. 2.This is accomplished using a new drilling pattern.

[0021] The first step, 500, is to separate the positions on materialsample 100 where holes are to be drilled into two groups. These groupsshould be chosen so that no two holes in a given group are close enoughto each other that the laser hardened region formed during the machiningof one hole will overlap part of the other hole. Mathematically thismeans that the separation between any two holes in the same group is atleast equal to the width of a laser hardened region. For a line of holesthe groups may usually be selected by numbering the holes sequentiallywith the even numbered holes being one group and the odd-numbered holebeing the other. It is contemplated that, if the holes of one group arestill too close together after separating the holes into two groups,then three group may be created.

[0022] At the next step 502, the pulse energy of the laser beam is setto a level that provides a laser beam intensity within the central areaof focused spatial profile of the laser beam which is above the ablationthreshold of the unhardened material. The area of the laser beam profilein which the intensity exceeds the ablation threshold is desirably thesame as the area of one of the holes to be machined.

[0023] Next, in the exemplary embodiment shown in FIG. 5, the sample ispositioned such that the laser beam is focused on one of the first groupof hole positions on the surface of the sample, step 504. The sample isprecisely position in the X and Y directions to properly position thehole on the surface, and in the Z direction to focus the laser beam onthe surface. It is contemplated that the long working length microscopeobjective 202 may be manipulated to focus and/or position the laser beaminstead. It may be desirable for the laser beam to be propagated betweencomponents of the laser machining apparatus within an optical fiber inthe case when the microscope objective is moved to position the laserbeam.

[0024] A predetermined number of pulses are applied to ablate thesurface 506. The number may be calculated to introduce a desiredhole-size on non-hardened area. Alternatively the hole drilling processmay be monitored in situ to ascertain when an appropriate number ofpulses have passed. In this way, very high accuracy in hole depth may beattained.

[0025] The decision at step 508 causes the previous two steps, 504 and506, to be repeated until all of the holes in the first group have beendrilled.

[0026] Once all of the first positions on the surface have a hole, theprocess moves to step 510. The pulse energy of the laser beam is set toa second level that provides a laser beam intensity above the ablationthreshold of the laser hardened material. The area of the laser beamprofile in which the intensity exceeds the ablation threshold isdesirably the same as the area of one of the holes to be machined.

[0027] Next, the sample is positioned such that the laser beam isfocused on one of the second group of hole positions on the surface ofthe sample, step 512. The sample is positioned as in step 504.

[0028] The a number of pulses of laser beam are then applied to ablatethe surface 514. The number may be calculated to introduce a desiredhole-size on the laser hardened area or alternatively it may bemonitored as previously described with regard to step 506.

[0029] These last two steps, 512 and 514, may be repeated until all ofthe holes of the second group have been drilled. If there is a thirdgroup of hole positions, the pulse energy may be increased again toaccount for the material being twice-exposed to the laser beam and thethird group of holes drilled in a manner similar to the first twogroups.

[0030] This method is applicable to any laser machining process wherelaser hardening of the surrounding areas may be present and may preventreliable drilling results when employing prior drilling sequences. Anexemplary implementation of the present invention, illustrated in FIG.3, involves drilling holes 102, with femtosecond pulses, in substrate300 to fabricate a photonic crystal. This substrate may be desirablyformed of a dielectric material or a multi-layer structure, such as asilicon-on-silicon dioxide structure. The exemplary photonic crystalillustrated in FIG. 3 has been formed as a two-dimensional photoniccrystal structure with the holes arranged in a square pattern. Thesquare pattern is only one possible pattern for a two-dimensionalphotonic crystal. Other patterns, such as a hexagonal pattern, may alsobe formed using the present laser machining method. It is contemplatedthat a one-dimensional photonic crystal structure may be formed usingthe laser machining process of the present invention, as well.

[0031]FIG. 4 illustrates an additional application of the exemplarylaser machining process described above with reference to FIG. 5. Thematerial sample 100 in FIG. 4 has had a groove 402 laser machined intoits surface. This groove may be cut according to the present inventionby forming a series of sub-wavelength diameter holes having a pitchwhich is a fraction of a hole diameter, so that the holes overlapforming a substantially smooth groove. To form such a groove, it may bedesirable to separate the hole positions into at least three groups.

[0032] While the invention has been described in terms of an exemplaryembodiment, it is contemplated that it may be practiced as describedabove within the scope of the appended claims. Also, it will beunderstood to one skilled in the art that a number of othermodifications exist which do not deviate from the scope of the presentinvention as defined by the appended claims.

What is claimed:
 1. A method to drill a plurality of holes in a materialusing an ultra-fast pulsed laser beam, comprising the steps of: a)setting a pulse energy of the laser beam to a first predetermined level,selected to provide an intensity greater that an ablation threshold ofthe material within a hole-drilling portion of the laser beam; b)positioning the material to focus the laser beam on one of a pluralityof first positions on a surface of the material; c) applying a number ofpulses of the laser beam to ablate the surface, thereby forming one ofthe plurality of holes in the surface; d) repeating steps b) and c)until all of the plurality of first positions on the surface have one ofthe plurality of holes; e) setting the pulse energy of the laser beam toa second predetermined level, selected to provide an intensity greaterthat a laser hardened ablation threshold of the material within thehole-drilling portion of the laser beam; f) positioning the material tofocus the laser beam on one of at least one second position on thesurface, wherein the one second position is between two adjacent ones ofthe plurality of first positions; and g) applying a number of pulses ofthe laser beam to ablate the surface, thereby forming one of theplurality of holes in the surface.
 2. A method according to claim Iwherein the at least one second position includes a plurality of secondpositions, each of the plurality of second positions being between twoadjacent ones of the plurality of first positions, the method furthercomprising the steps of: h) repeating steps f) and g) until all of theplurality of second position on the surface have one of the plurality ofholes; i) setting the pulse energy of the laser beam to a thirdpredetermined level, greater than the second predetermined level; j)positioning the material to focus the laser beam on one of at least onethird position on the surface, wherein the one third position is betweenone of the plurality of first positions and an adjacent one of theplurality of second positions; and k) applying a number of pulses of thelaser beam to ablate the surface, thereby forming one of the pluralityof holes in the surface.
 3. A method according to claim 1, furtherincluding the step of operating the laser such that the laser beamoscillates in the TEM_(0,0) mode.
 4. A method according to claim 1,wherein the laser beam has a wavelength and the step of setting thepulse energy of the laser beam to the first predetermined level includesthe step of setting the pulse energy of the laser beam to define thehole-drilling portion of the laser beam to have a diameter less than thewavelength of the laser beam.
 5. A method according to claim 4, whereinthe laser beam has a wavelength of 775 nanometers and the step ofsetting the pulse energy of the laser beam defines the hole-drillingportion of the beam to have a diameter of approximately 500 nm.
 6. Amethod according to claim 1, wherein the step of setting the pulseenergy of the laser beam to the second predetermined level includes thestep of setting the second predetermined level of pulse energy to alevel that is greater than the first predetermined level of the pulseenergy.
 7. A method according to claim 1, wherein the step of settingthe pulse energy of the laser beam to the first predetermined levelincludes the step of setting the pulse energy to be between 1 nanojouleand 1 microjoule.
 8. Apparatus for drilling a plurality of holes in amaterial, comprising: an ultrafast pulsed laser which provides a laserbeam; means, coupled to the ultrafast pulsed laser, for setting a pulseenergy of the laser beam to a first predetermined level, selected toprovide an intensity greater that an ablation threshold of the materialwithin a hole-drilling portion of the laser beam; a translation tablewhich positions the material to a first plurality of positions to focusthe laser beam at each of the first plurality of positions on a surfaceof the material; means for applying a number of pulses of the laser beamto ablate the surface at each of the first plurality of positions toform a first portion of the plurality of holes in the surface; means forsetting the pulse energy of the laser beam to a second predeterminedlevel, selected to provide an intensity greater that a laser hardenedablation threshold of the material within the hole-drilling portion ofthe laser beam; means, coupled to the translation table for positioningthe material to focus the laser beam on one of at least one secondposition on the surface, wherein the one second position is between twoadjacent ones of the first plurality of positions; and means forapplying a number of pulses of the laser beam at the secondpredetermined level of beam energy to ablate the surface at the onesecond position, thereby forming one of the plurality of holes in thesurface.
 9. Apparatus method according to claim 8 wherein the at leastone second position includes a plurality of second positions, each ofthe plurality of second positions being between two adjacent ones of theplurality of first positions, the apparatus further comprising: meansfor setting the pulse energy of the laser beam to a third predeterminedlevel greater than the second predetermined level; means for positioningthe material to focus the laser beam on one of at least one thirdposition on the surface, wherein the one third position is between oneof the plurality of first positions and an adjacent one of the pluralityof second positions; and means for applying a number of pulses of thelaser beam to ablate the surface of the material at the one thirdposition, thereby forming one of the plurality of holes in the surface.10. Apparatus according to claim 8, wherein the laser beam oscillates inthe TEM_(0,0) mode.
 11. Apparatus according to claim 8, wherein thelaser beam has a wavelength and the energy level of the laser beam isset to define the hole-drilling portion of the laser beam to a diameterless than the wavelength of the laser beam.
 12. Apparatus according toclaim 11, wherein the laser beam has a wavelength of 775 nanometers andthe hole-drilling portion of the beam to have a diameter ofapproximately 500 nm.
 13. Apparatus according to claim 8, wherein thesecond predetermined level of pulse energy is greater than the firstpredetermined level of the pulse energy.
 14. Apparatus according toclaim 8, wherein the pulse energy of the laser beam is between 1nanojoule and 1 microjoule.
 15. A photonic crystal comprising asubstrate having a plurality of holes formed by the method of claim 1.16. The photonic crystal of claim 15, wherein the substrate includes adielectric material.
 17. The photonic crystal of claim 15, wherein thesubstrate includes a multi-layered dielectric material.
 18. The photoniccrystal of claim 17, wherein the multi-layered dielectric materialincludes a silicon layer and a silicon dioxide layer.
 19. The photoniccrystal of claim 15 wherein the plurality of holes is a linear array ofholes and the photonic crystal is a one-dimensional photonic crystal.20. The photonic crystal of claim 15 wherein the plurality of holes is amatrix of linear holes and the photonic crystal is a two-dimensionalphotonic crystal.
 21. A method of drilling a plurality of holes in amaterial using a laser machining apparatus including an ultra-fastpulsed laser and an output coupler through which a laser beam passes,comprising the steps of: a. setting a pulse energy of the laser beam toa first predetermined level; b. positioning the output coupler to focusthe laser beam on one of a plurality of first positions on a surface ofthe material; c. applying a number of pulses of the laser beam to ablatethe surface, thereby forming one of the plurality of holes in thesurface; d. repeating steps b. and c. to position the output coupler atrespectively different ones of the first position until a portion of theplurality of holes have been formed at the plurality of first positions;e. setting the pulse energy of the laser beam to a second predeterminedlevel; f. positioning the output coupler to focus the laser beam on oneof at least one second position on the surface, wherein the one of theat least one second position is between two adjacent ones of theplurality of first positions; g. applying a number of pulses of thelaser beam to ablate the surface, thereby forming one of the pluralityof holes in the surface.
 22. A method to drill a plurality of holes in amaterial using an ultra-fast pulsed laser beam, comprising the steps of:a) setting a pulse energy of the laser beam to a predetermined level,selected to provide an intensity greater that an ablation threshold ofthe material within a hole-drilling portion of the laser beam; b)positioning the material to focus the laser beam on one of a pluralityof first positions on a surface of the material; c) applying a firstpredetermined number of pulses of the laser beam to ablate the surface,thereby forming one of the plurality of holes in the surface; d)repeating steps b) and c) until all of the plurality of first positionson the surface have one of the plurality of holes; e) positioning thematerial to focus the laser beam on one of at least one second positionon the surface, wherein the one second position is between two adjacentones of the plurality of first positions; and f) applying a secondpredetermined number of pulses, greater than the first predeterminednumber, to ablate the surface, thereby forming one of the plurality ofholes in the surface.
 23. A method according to claim 22 wherein the atleast one second position includes a plurality of second positions, eachof the plurality of second positions being between two adjacent ones ofthe plurality of first positions, the method further comprising thesteps of: g) repeating stews e) and f) until all of the plurality ofsecond position on the surface have one of the plurality of holes; h)positioning the material to focus the laser beam on one of at least onethird position on the surface, wherein the one third position is betweenone of the plurality of first positions and an adjacent one of theplurality of second positions; and i) applying a third predeterminednumber of pulses of the laser beam to ablate the surface, the thirdpredetermined number being greater than the second predetermined number,thereby forming one of the plurality of holes in the surface.
 24. Amethod according to claim 22, further including the step of operatingthe laser such that the laser beam oscillates in the TEM_(0,0) mode. 25.A method according to claim 22, wherein the laser beam has a wavelengthand the step of setting the pulse energy of the laser beam to thepredetermined level includes the step of setting the pulse energy of thelaser beam to define the hole-drilling portion of the laser beam to havea diameter less than the wavelength of the laser beam.
 26. A methodaccording to claim 25, wherein the laser beam has a wavelength of 775nanometers and the step of setting the pulse energy of the laser beamdefines the hole-drilling portion of the beam to have a diameter ofapproximately 500 nm.
 27. A method according to claim 22, wherein thestep of setting the pulse energy of the laser beam to the predeterminedlevel includes the step of setting the pulse energy to be between 1nanojoule and 1 microjoule.